The invention relates to a pulverulent zirconium oxide containing metal oxides from the group consisting of scandium, yttrium, rare earths and mixtures thereof, processes for producing them and their use in fuel cells, in particular for the production of electrolyte substrates for ceramic fuel cells.
Pure zirconium oxide (ZrO2) exists in three modifications. The cubic high-temperature phase is converted at below 2300° C. into metastable tetragonal zirconium oxide, and the transformation from tetragonal into monoclinic ZrO2 is observed between 1200° C. and 950° C. Transformations between the monoclinic phase and the high-temperature phases on heating and cooling are associated with step changes in volume.
Sintering of zirconium dioxide occurs in a temperature range which is significantly above the temperature of the reversible monoclinic-tetragonal phase transformation. To avoid conversion back into the monoclinic phase, the high-temperature modifications have to be stabilized by means of foreign oxides. The stabilized zirconium oxides are then present in the same stabilized modification from room temperature up to the melting point, i.e. the large volume changes on cooling in the manufacture of ceramic components are avoided, see Ullmann's Encyclopedia of Industrial Chemistry, Vol. A28, 1996, p. 556 ff., Römpp Lexikon Chemie, 10th edition 1999, p. 3073. Stabilized or partially stabilized zirconium oxide powders are therefore used for producing ceramic components. The stabilizer oxides have to be able to form a solid solution with zirconium oxide. This requirement is met when using alkaline earth metal oxides, scandium oxide, yttrium oxide and some oxides of the lanthanides and actinides. The amount of stabilizer required depends on the desired properties and the type of oxide.
Unsatisfactory homogeneity of the stabilizer in the ZrO2 lattice leads to the presence of proportions of undesirable monoclinic, i.e. unstabilized, phases. Depending on the concentration, type and amount of stabilizer oxides and the sintering conditions employed, tailored zirconium oxide materials having improved properties can be produced and these are used, for example, in structural elements and components in modern mechanical engineering, in human medicine, in cutting tools and in thermal insulation layers.
In recent years, zirconium oxides doped with yttrium oxide have been increasingly used in the production of ceramic fuel cells. An important property of substrates produced from zirconium oxides for ceramic fuel cells is their electrical conductivity, which has a critical influence on the performance of the fuel cell.
According to WO 03/051790, stabilized zirconium oxides are usually produced by two main methods in different variants.
In wet-chemical methods, solids containing both zirconium and stabilizer metals are separated from aqueous or organic solutions or suspensions of zirconium precursors and stabilizer precursors. In general, the solids are separated off by means of coprecipitation and filtration of the hydroxides. However, other separation techniques such as sol-gel, evaporation, spray pyrolysis and hydrothermal processes are also employed. After separation of the precipitated precursors, these are then calcined at temperatures in the range from 500 to 1500° C.
U.S. Pat. No. 3,957,500 describes a coprecipitation process for producing a homogeneous mixture of zirconium hydroxide and yttrium hydroxide. After calcination at from 900 to 1500° C. for from 1 to 10 hours, the stabilized zirconium dioxide is formed.
A similar typical commercial process is described in U.S. Pat. No. 4,810,680, in which basic zirconium carbonate and yttrium carbonate are dissolved in hydrochloric acid. The hydroxides are subsequently coprecipitated by addition of ammonia or sodium hydroxide. The hydroxide mixture is washed, dried and calcined at from 680 to 980° C.
DE 10138573 discloses a nanosize pyrogenically produced tetragonal yttrium-stabilized zirconium oxide (YSZ) powder and the process for producing it. Here, aqueous and/or alcoholic solutions of Zr and Y precursors, e.g. nitrates and propionates, are atomized by means of a nozzle in a reaction tube in which a hydrogen/air flame is burning and subsequently burnt at temperatures of from 800 to 1000° C.
U.S. Pat. No. 5,750,459 describes the production of gels or spherical or microspherical particles by dropwise introduction of a Y/Zr nitrate solution into an ammonium hydroxide solution. After separation and rinsing of the gels or agglomerates produced with water and subsequent calcination at temperatures above 550° C., spherical and microspherical stabilized zirconium dioxide powders are obtained. The high filtration rate of gel precursors is a decisive disadvantage over the traditional hydroxide precipitation process.
A disadvantage of all wet-chemical processes described is the large amounts of wastewater obtained. In addition, it is always necessary to carry out elaborate washing steps in order to remove all by-products. If washing is incomplete, offgases such as HCl/Cl2 or NOxare formed during calcination of the precursors.
The other method of producing stabilized ZrO2 powders is the mixed oxides or solid-state process. In this process, mixtures of zirconium dioxide and stabilizers are homogenized and subsequently sintered to form stabilized ZrO2 powder. The solid-state process is simple and inexpensive to carry out. In contrast to the wet-chemical processes, no by-products or polluted wastewater and offgases, apart from recyclable water or water vapor, are formed.
Disadvantages of the process are the high sintering temperatures of >1300° C. and the low homogeneity of the powders, which contain from 25 to 30% by volume of monoclinic phase after sintering. To minimize the proportions of monoclinic phase, the products are repeatedly milled and heat treated in a plurality of stages, resulting in a significant increase in the product costs. Stabilized ZrO2 powders are therefore rarely produced by the mixed oxides process.
U.S. Pat. No. 4,542,110 discloses a process for producing a sintered body via wet milling of a mixture of zirconium dioxide and yttrium oxide with addition of SiO2 and Al2O3 as sinter aids and subsequent drying and sintering of the mixture at temperatures of >1300° C., preferably in the range from 1400° C. and 1500° C., for from 10 to 120 minutes. After subsequent repeated mixing and heat treatment, the proportion of cubic phase is increased to at least 95% by volume.
U.S. Pat. No. 4,360,598 describes a process for producing a YSZ ceramic by mixing of amorphous zirconium dioxide with yttrium oxide or an yttrium-containing salt and subsequent sintering. After sintering at temperatures of from 1000 to 1550° C., ceramic bodies which comprise mainly tetragonal and cubic zirconium dioxide are obtained.
EP 1076036 describes the production of zirconium oxides stabilized with yttrium or other metals by melting of the precursors in high-frequency or medium-frequency furnaces at temperatures of from 2200 to 3000° C.
DD 96467 discloses a fully stabilized cubic zirconium dioxide which is produced by mixing of basic zirconium carbonate and stabilizing additives such as calcium oxide or yttrium oxide and subsequent sintering at 800° C/3 h.
WO 03/051790 describes a process for producing tetragonal zirconium dioxide or mixtures of tetragonal and cubic zirconium dioxide.
A disadvantage of the zirconium dioxide powders produced according to the prior art via the mixed oxides process is their unsatisfactory homogeneity of the stabilizers in the crystal lattice. For sufficient stabilization nevertheless to be ensured, high sintering temperatures are necessary. However, these lead to higher production costs, at least in part due to the additional process steps required (crushing, classification). Furthermore, the high sintering temperatures lead to undesirably low BFT values and to reduced sinter activity of the powders. These powders are unsuitable for use in ceramic fuel cells because of their low electrical conductivity and unsatisfactory sinter activity.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide zirconium oxide powders for use in ceramic fuel cells, which have high electrical conductivities and high mechanical strengths after having been sintered to gastight bodies.
A further object of the present invention is to provide an economical process for producing the zirconium oxide powders.
The object is achieved by a pulverulent zirconium oxide containing up to 10 mol % of at least one metal oxide from the group consisting of scandium, yttrium, rare earths and mixtures thereof which has a fill density of from ≧1.2 to 2.5 g/cm3, measured in accordance with ASTM B 417.
The pulverulent zirconium oxides of the invention preferably have a fill density of from ≧1.2 to 2.3 g/cm3, particularly preferably from ≧1.6 to 2.0 g/cm3, particularly preferably from ≧1.3 to 1.9 g/cm3 and in particular from 1.5 to 1.7 g/cm3. The zirconium oxides of the invention preferably have a fill density of from ≧1.5 to 2.5 g/cm3, particularly preferably from ≧1.6 to 2.3 g/cm3.